Para xylene separation process



March 16, 1954 Filed May 28, 1949 B. R. TEGGE ET AL PARA XYLENE SEPARATION PROCESS 2 Sheets-Sheet l A l l H) L; L') 7 Z "\I Q .J O I l b. 0 i

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U1 U] @l Ii 1 5 X 2 w-( F I 6 e T u] 5 5 dc 2. T ;e Carl 5. Cal Eg Joseph Stewart {inventors b MG L C(btorneg Patented Mar. 16, 1954 UNITED LATJENT fQFF ICE PARATXYLENESEPARATIONPRCFCESS xliBrucesRtfllegge-"and Carl S..Garlson, Roselle, N.-J., i and 'J osephsstewart, Brooklyn N. vY., --assignrs to StandardrOiLDeveIOpment.Company, a corporation-oflDelaware -App1icatiomMa'y 2s, 1949; Sei'ial-No. 96,'058

eolaims. (c1.' 260-i674) :This "invention relatesto ttheiseparatlon xof *havinga boiling "pointin the-rangev of 130'-160 para xylenefrorri hydrocarbon= mixtureslcontain- -C. This-.feed'is fed through line I and heat ine'para-xyleneand its isomers. :exehanger fi andthemthrough -1ine.-3 intoa cyiiylenes' are obtained:commercially'from r-varit lindrical scraped-surface chiller 4 whichoperates =cus natural sou1 ces, 'such as LookeLoVenxdistil- 5 -.at a lowscraping:frequency.ofsapproximately 5 dates, as -Well "as from other sources-such as hy $to.1:0- scrapes'periminute. :Inthe scraped-surdrocarbon-cracking and hydroform'ing xrperarace chiller i; themixtureis cooledto about 60 tions. A xylene fraction thus obtained "ordinarily "0., atemperature abovetthe eutecticlpointsso contains an three xylenes; as-wel1 as ethyllzbenithat-the para xylene deposits from the mother -A reasonably -efiective separation of ortho :liquor aresubstantially:free of. othercomponent xylene from this mixture-might be-'made -by=' frac crystals. *Thexexact temperature of crystallization "tionation-"of thexylene mixture. Meta anw para is a function not only of the para xylene conxylenes,howeven-are sm close in boiling points :centration, but also iofthe relative proportion that fractionation-= is inff-ectiveto br ing about of the other isomers present.

separation. "Low temperature*separation by '15 Operation of this' scrape'd-surface-chiller 4 at crystallizationpffpara 'x-ylene' i -complicated 'by thetlow scraping frequenciesi; e., '5 to and theformation of-=eutectic mixtures which result preferably '5 to -10 scrapesperminute, results "in-mixed-product=and consequent low yields of .inthe-production of crystalline para'xylene of the purer products. a relatively large size which facilitates handling Thus UsS. -Pate'nt' No. *129401065-teach-es that 0 rinfurther operations and also contributes to betprior to'the crystallization*of thepara xylene, ter 'overall yields and rate of crystallization. from a fee'd'containing 'a number df Ca -isomers, ficraping frequencies refer-to the number of times it is necessary to firstdistilPcfimrd*remove alia scraping blade passes a-rfixed pointonthe 'wall phatic hydrocarbons, -=ethy1 benzeneand-crtho ofzthe chil'ler per minute. Thus the expressed "xylene. The "para- *xylene *is them-crystallized 5 :frequencies are equiva'lent to-the RrP. M..of a

'i'romta miz-ztureconsisting essentially of onlypara single helical screw -ior blade. Use of adouble "xylene and metaxylene. fhelical "sorewor a two-blade scraper would re- IThisinvention"comprises an improved low tem- :duce the R. :P. by one-halfand-stilLgive-the Qp p ofi s for'fihe p on-"of-pure'para desirediscra'ping frequency. The optimum-scrap- "xylenelrom a hydrocarbon*mixture'containing ing frequencyis 5"to l0 scrapesperminute.

'at least' one of its. isomers"arrd-'usually more than The slurry fcontaining the large para xylene 'one,"withont the 'necessity"for"first removing crystals suspended in the mother liquor .is-disany of "the "hydrocarbons. ncccording towhe charged throughllinei' into a holding tank 6 metho'd f is inventifln; p axylene' 'is'c y equipped -With=agitat0r 1.31. The holding tank 6 tallized from its mother liquor atatemperature is maintainediat approximately th z -.1; "above'the'eutecticpointsofthe feed themrystal perature as the e'fiiu'ent: from chiller s byrefrigd" ba a Xylenvalidritswom eratedjacket w. 'Tl'le slurryrremains in theaagisequent yield is regulated by control led' scraping tated holding tank 'for a 'm'inimum' residence 'frequenciesandcontrolledcrystal growth time, time-of abouttflrminutesrfor the desired-crystal andithe"I ra XyIenecryStaISthusdbtained'may -growth. Approximately the 'imaximum "amount be finally Washed, most advan ly, 'withvan of para. xylene obtainable :isnobtained wvithin a immiscible liquid ino'rde'r to further" purify-"them. "tw -h period,

This mventiomwfll'be better 'unde'rstoodby Alternatively,a uiajoffportioin"of the feedrcan re'ferencetothe fiowdiagramshown"in thedrawbe rapidly 'passeddirectl iint hoiding'iijank e 'ing,'Figuresil"andvzf-Fieur '2 isa 'continuation 5 through refrigerated coolers 4| .and-s l-zvand of Fi ure i1 andlines!,-"33 and fi re-"shownin *through lines ltl and Mia-Where theliquidtissuboth'figures. "percooled to I approx-ima'tely the same "tempera- "The *feed" xyleneytyp'ically are 'a*- mixture of -ture as 1 in --'scraped-'surface chiller 4. A wminor *orthofmeta, "and para xylen'es and ethyl henproportion -:ofi:theefeed sis iconcurrently lpassed "zena'such-as"an*aromatic hydrocarbon iraction poi'ithrough the scrapedesurfacetlchiller 4 which is operated in the same manner as discussed above. The crystal nuclei which are formed in the scraped-surface chiller are discharged through line 5 into holding tank 6 and act as centers for deposition of the supercooled para xylene. The size of the crystals obtained depend on the ratio of para xylene in the supercooled liquid to the number of nuclei present. For a given amount of para xylene deposited at equilibrium, the size of the crystals varies inversely with the number of nuclei. By splitting the feed stream, and independently maintaining a low scraping frequency, it is possible to obtain larger crystals than has hitherto been possible by the use of conventional techniques. It is therefore desirable to produce smaller numbers of large nuclei rather than large numbers of smaller nuclei.

As still another alternative, the para xylene crystallization can be conducted in a large scraped-surface vessel containing an agitator in addition to a scraper, thus consolidating equipment units t and S. In this case, in addition to the scraping action, which is conducted at the same rate as previously, i. e., about 5 to scrapes per minute, it is necessary to maintain agitation at a value such that the peripheral speed imparted to the liquid mass does not give the effect of increased scraping frequency. This, in effect, will be determined by the size and geometry of the vessel and agitator. Thus it was found that for an ordinary 2 diameter propeller type agitator in a 5" diameter by 9 cylindrical vessel, it was preferable to keep the agitation frequency below 609 R. P. M. In these operations various portions of the liquid can be recirculated into contact with the chilled surface from which the deposited crystals are slowly scraped and then suspended in the bulk of the agitated liquid.

It is important, therefore, in general, to have a low liquid speed and a low scraping speed at the point of crystal nucleus formation which is largely at the wall of the crystallizer. For maximum yield per unit time, it is important to maintain a higher degree of agitation in the main and central portion of the liquid mass than is maintained in the zone of crystallization formation as explained above.

The slurry is discharged from holding tank '5 either continuously or in batches through line I to a centrifugal separator 8 which may be a batch centrifuge or a battery of batch centrifuges or a continuous perforated centrifuge, a continuous solid bowl centrifugal separator such as the Bird centrifugal filter or the Sharples Super D-Canter, or similar centrifugal equipment.

In a batch centrifuge operation, here described, the mother liquor is removed from the crystals through line 33 and is heat exchanged in heat exchanger 2 with the incoming feed xylenes to enable more economic utilization of refrigeration capacity. The residual mother liquor, which has had a large proportion of para xylene removed, may be used for other operations or added directly to aviation fuel.

Although hydrocarbon miscible solvents are known to have been employed to wash crude para xylene crystals and could be employed on the separated para xylene cake from centrifuging means 8, it is particularly advantageous to employ immiscible wash liquids as described hereafter. This is especially true when large para xylene crystals as taught above are being washed.

The crystals of para xylene in the centrifuge 8, which are in the form of a centrifuge cake, are thus then washed with an aqueous, relatively immiscible wash liquid which is liquid at the temperature in the centrifuge, e. g., at about 58 C. The wash liquid is employed in a ratio of about 10 to 100 parts and preferably to 50 parts of wash liquid per 100 parts of centrifuge cake. The exact limits are a function of the degree of saturation of the wash liquid, temperature and composition of the wash liquid and the product purity desired. Such a wash liquid conveniently is aqueous methanol containing 20% water. This wash liquid, which may be at a temperature of 58 6., enters centrifuge through lines 52 and 9. The immiscible wash liquid is then separated from the para xylene cake by centrifugal action.

Due to the immiscibility of para xylene in the wash liquid, any liquid impurities held on the crystal surfaces are displaced and washed away. Because of the higher temperature of the wash liquid, any eutectic composition which may have inadvertently deposited with the pure para xylene, is melted and washed away. The melted hydrocarbons are returned to feed so no losses in yield are thereby incurred. The wash liquid containing hydrocarbon impurities and any para xylene is passed through line Ill to a settler H. In the settler, two phases are formed: a lower phase consisting of hydrocarbons and an upper wash liquid phase. The wash liquid is recycled through lines 12 and 9 and reused for washing in centrifuge ii.

The hydrocarbon phase is withdrawn through line It into an orifice mixer Hi with warm water injection from line 15. From the orifice mixer i l, the mixture of water, hydrocarbon and methanol is passed through line 88 to a settler it where the hydrocarbon phase is removed through line ll and recycled to feed. The aqueous phase containing some methanol is sent through lines l8 and 19 to a methanol stripping tower 28 where water is rejected through line 2!. Methanol is recycled through line Ill, condenser 44, lines 22 and t5, cooler 46 and line 9 as makeup for the wash liquid. The composition of the water-- methanol wash liquid is controlled by the amount of methanol reflux in line 36.

The para xylene cake in centrifuge B is dumped through line 23 into a melt tank 24, where it is heated to above its melting point of 13.3 C. This liquid para xylene, withdrawn through lines 25 and ti, is scrubbed with water from line 21 in an orifice mixer 26. Warm liquid para xylene is returned through line 532 to melt tank 26 in order to melt additional solid para xylene. A mixture of para xylene, water and alcohol, is withdrawn from mixer 26 through line to settler 28, from which the pure para xylene phase is removed as product through line 29. The aqueous phase from the settler containing some methanol is sent to the methanol stripper 21': through line 19.

Slow scraping frequencies in chiller 4, i. e., a frequency of about 5 to 10 scrapes per minute, greatly increases the para xylene crystal size. Thus, for example, when the chiller was operated at normal scraping frequencies of about 100 scrapes per minute, crystals of approximately microns length were obtained. When the scraping frequency was reduced to 10 scrapes per minute, the crystal size increased to 300 microns, a six-fold increase. This large increase in crystal size also contributes to improved deposition of crystals in holding tank 6 by acting as nuclei for further "crystal growth. This large crystal size "ma kes possible the obtaining of high yields of very pure product.

The crystallizationof the *p'araxylene "from the mother liquor is'conductedabove; the eutectic points of the feed in order to prevent deposition of mixed impure product. These eutectic points, i. e., the temperature at which the eutecticsbegin to crystallize from solution,"vary withthe "composition of the-feed material and can be calculated for any compositions of the naturedescribed. Sometypical 'first' eutectic points are iven in Table I- below.

Typical feed compositions as well as the eutectic'points above which the processis'operated,

are indicated in Table I.

--Para Xylene Crystallization ;Point, C.

First Eutectic Point Feed (Wt. Percent) I and-Components 39.5% ortho xylene. 31.6% meta xylene... 7.9% ethyl benzene... [21% ortho xylene 48% meta xylenc 21% para Xylene I 55 C. (para/ortho).

67 0. (para/m eta).

- 12% ethyl benzene I 60.8% meta xylen III. 24.1% para xylene 15.1% ethyl benzcne IV {71.6% meta xylene.-."

u 59 C. (para/ meta).

52 C. (para/meta).

23.9% ortho xylene V 4.6% meta xylene 36 63 C. (para/metal 21.5%pura xylene.-. 'ortho).

The feed therefore is not cooled to a tempera ture at which the para xylene crystallizes until the feed enters scraped-surface chiller 4 or coolers ll and 42.

While the above-listed feed compositions consist of C8 aromatic isomeric mixtures, suitable feed stocks may also contain small amounts of paraflinic and naphthenic'hydrocarbons of the appropriate boiling point range, i. e., 130-l60 C.

In general, therefore, the crystallization temperatures employed in the scraped-surface chiller vary in the range of about 40 C. to about 90 0., and most advantageously 60 .C. to 70 .0. when ethyl benzene is present with the V-three xylene isomers.

Preferred sources of the feed mixtures-for-the crystallization are highly aromatic distillate fractions obtainedfrom aromatic solvent extracts of catalytically cracked, hydroformed or thermally cracked petroleum oils.

These feed mixtures yield close to 10 wt. percent or more of pure xylene when the feed cut is properly made and the separation process is properly controlled.

Quite different from what would be'expected ,onthe basis of other types of separation than the process of this invention, the best recovery of para xylene is obtained when the feed mix- ;ture.is a wider cut fraction than is taught in the priorart, i. e., one that includes a substantial -proportion ofethyl benzene (B. P. -136.2 C.),-as

shown in the feed II of Table I. Thus, with a fraction containing the three xylenes, including q-ortho xylene, it is desirable to have theinitial boilinglpoint ofthefraction below;136.2 0., e. e.,

at about 130 0., so that the ethyl benzene amounts to at least 10 wt. percent of-. Ca aromatic hydrocarbon components. The. crystallization .s'lurryof this kind of .feed .can be safely cooled down to below 60 0., preferably between 5 6 C. and 70'C.,for crystallizing'outsubstantially pure para xylene, even though 'these temperatures are'belowthenormal crystallization point of orthoxylene and below the normal binaryand ternary eutectic points of ortho xylene withthe meta" and para xylenes.

The holdup time and agitationirr holding-tank *6 has alsobeen found to be important in heterminingtheyields secured. Thus a feed composition containing 15.4%- para xylene, -39.5-% meta xylene, 20.5% --ethyl benzene; and i 22.4% '--ortho xylene, all volume percent, was processedaccording to the method of this invention. --The results are--presente-d below in Table II. i he-purities --were*--obtained by infra -red= spectrographic =or cryoscopic analysis and their high-levels should -be-noted. A minimum holdup -time-of about 30 minutes is also indicated from-the data,=aswell as adequate stirring inthe holding tank.

Table. II

Improvement of Yleld Yield by Stirring gvithout xPzira 390 PM PAM .tirring, yeno 1 ,Iloldmg Fune (min) VOL Pep y cel letegn Percent n i m 2 6.7 95.5 119 8.2 I 94.8 -120 as taco I 111 9.1 94.3 103 8.8 95.3 118 -9.9 H

The preferred immiscible --wash -rl-iquids-are water-methanol compositions which may vary in the' range of 5 to 50parts-of water and from 50 to-95 parts of-methanol. The'usaof- 100% water compositions as the wash liquid-would result in the crystallization-of thelatterat the low washing temperatures employ-ed.

Water-methanol solutions containing up -.to

about 20%- water have lower specific gravities .than Caaromatics, evenat/room temperatures, and thus appear as the-upper phase in settler -H.- Gn-the other hand, when the immiscible methanol wash liquids contain 25% of watenthe wash liquids have higher specificgravities-than the hydrocarbon phase insettler li at-rcom temperatureswbut becomelighter than the latter [as-the temperature diminishes to '6i1- C Ti; is desirableto have the hydrocarbon clhasea-as 1 :the' lower phasein settler H to preventphase-inversion due to changesin temperature during the operation. This can: lie accomplished by'using a composition containing at least-% -methanol. A 20% water-86% methanol solution is therefore particularly adapted for use.

Other suitable relatively immisciblewash liquid compositions consist of mixtures of water.

2. Solution of para xylene impurities by the organic constituents of the wash liquid.

3. Melting of low melting para xylene impurities by the sensible heat of the immiscible wash liquid.

Only the first two of these three factors are operative when the wash liquid is introduced on the para xylene cake at a temperature at or below the temperature of the latter. The third factor becomes operative when the wash liquid is introduced at a temperature above the temperature of the para xylene cake. The upper temperature limit of the wash liquid is therefore dependent upon heating economics and the amount of para xylene it is desired to melt in the para xylene cane.

Ihe characteristics necessary for the lower molecular weight polar organic oxygenated compounds to be used with water in this operation as wash liquids are:

1. Relative immiscibility with the C8 aromatic hydrocarbons at the temperatures of operation.

2. Low viscosity at the temperatures of operation.

3. A crystal point below the temperatures of operation.

4. Complete miscibility with water at temperatures at which the wash liquids are employed.

It should be under-stood that whenever the term immiscible wash liquids is used, it refers to compositions of water and t indicated type of lower molecular weight polar organic oxygenated compounds which posses; the characteristics described above.

There are several advantages accruing from the use of the immiscible wash liquids. Thus, prior art processes required is lining of the para xylene centrifuge cake for periods of over two hours in order to obtain itylenes of 95% purity. The use of an immiscible wash liquid as taught in this invention permits the reduction of the washing time to the order of /2 to 10 minutes with increased product purities. The reduced frequency of scraping operation also contributes to the reduction of the necessary washing time.

When a miscible wash l eu such as isopentane, petroleum, ether, tolue or the like, is employed taught in the prior art, it becomes necessary to provide distillation facilities for the xylene filtrate, the xylene, and the wash liquid. Ihe size of the xylene filtrate necessitates an extremely large distillation or stripping tower. It is necessary, in addition, to strip the miscible wash liquid from the para xylene product in order to obtain adequate para Xylene puri is then also necessary to strip wan liouid filtrate in order to separate the wash liquid from the dissolved aromatic hydrocarbons. {Jperation of all these distillation facilities is uneconomical as compared to the process of this invention.

The use of the immiscible wash liquids of this invention permits the gravitational separation of impurities from the wash liquids. it also permits the settling of wash liquid entrained in the xylene filtrate or in the para xylene prod Where a wash liquid containing two com ponents, such as methanol and water, one of which is soluble in the xylene phase, is used, the partition coeificient is such that the methanol is readily removed from the para xylene by water scrubbing. This is much more economical than 8 distillation operations. In the process of this invention, only one stripping tower is employed to obtain methanol for recycling to the wash liquid. In a small plant, even this tower could be enminated and makeup methanol provided from an external source.

Analysis of the para xylene phase, from the para xylene melt tank 2a, which had been washed with an equal volume of a 75% methanol-25% water mixture, thus showed a content of only 0.2% wt. of water and 1.8% wt. of methanol. The methanol is then removed by a liquid-liquid scrubbing operation with water as shown in orifice mixer 26 and the residual water content in the para Xylene is still no higher than commercially acceptable.

It is to be understood that the invention is not limited to the specific embodiments which have been offered merely as illustrations. The para xylene crystals may thus be separated from residual mother liquid by other equipment than centriiuging, such as rotary filters, filter presses, vacuum or press filters, etc. Other modifications, such as auxiliary refrigeration facilities, scrubbing facilities, etc., may be made without departing from the spirit of the invention.

What is claimed is:

l. A process tor separating and purifying para xylene in a liquidpredominantly hydrocarbon feed boiling in the range of 130l60 C. which comprises the steps of initially crystallizing substantially only the para Xylene from the liquid ieed cold walls contacted therewith in a scraped-surface chilling zone at a temperature in the range of i(l C. to 90 C.; slowly scraping the walls of the scraped-surface chilling zone at a scraping frequency of about 5 to 10 scrapes per minute to remove relatively large crystals of para xylene therefrom; holding the thus scraped crystals in an agitated remaining portion of the liquid feed for a minimum holding time or thirty minutes to permit further para xylene crystal growth therefrom; and separating the obtained para xylene crystals from 2 the resulting mother liquor.

2. A process for separating para-xylene crystals from a liquid feed mixture thereof with meta-xylene, ortho-xylene and ethyl benzene which comprises continuously feeding a portion of the liquid mixture into contact with a chilled cylindrical wall surface inclosing a chilling zone to form crystal nuclei initially therein, chilling the liquid mixture in said chilling zone to a temperature in the range of 40 C. to -9G C. at which temperature the para-xylene is crystallized on the crystal nuclei adherent to said wall surface, slowly scraping the para-xylene crystals of relatively large size from said wall surface at a scraping frequency of about 5 to lo scrapes per minute to form a slurry of the crystals scraped from the wall in the remaining liquid contained in said chilling zone, continuously passing the resulting slurry from said chilling zone into a holding zone, agitating the slurry in the holding zone a temperature the range of i6 to -9c C. for a minimum holding time of thirty minutes so that the slurried para-xylene crystals continue to grow in size, maintaining a speed of agitation in the holding zone greater than in the chilling zone, and separating the remaining liquid from the slurried para-xylene crystals after they have grown in size by being held in said holding zone.

3. A process as in claim 2, in which another portion of the liquid feed mixture is precooled 9 without substantial crystallization and passed into said holding zone for crystallization on mixing therein with the slurry from said chilling zone.

BRUCE R. TEGGE. CARL S. CARLSON. JOSEPH STEWART.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,940,065 Spannagel et a1 Dec. 19, 1933 2,121,208 Milligan June 21, 1938 10 Number Name Date 2,398,526 Greenburg Apr. 16, 1946 2,428,102 Swietoslawski Sept. 30, 1947 2,435,792 McArdle et a1 Feb. 10, 1948 5 2,484,834 Keeling Oct. 18, 1949 2,541,682 Arnold Feb. 13, 19 51 OTHER REFERENCES Chemical Engineers Handbook, John H. Perry; 10 McGraw-Hill Book Company, N. Y.; 1941, pages 

1. A PROCESS FOR SEPARATING AND PURIFYING PARA XYLENE IN A LIQUID, PREDOMINANTLY HYDROCARBON, FEED BOILING IN THE RANGE OF 130*-160* C. WHICH COMPRISES THE STEPS OF INITIALLY CRYSTALLIZING SUBSTANTIALLY ONLY THE PARA XYLENE FROM THE LIQUID FEED ON COLD WALLS CONTACTED THEREWITH IN A SCRAPED-SURFACE CHILLING ZONE AT A TEMPERATURE IN THE RANGE OF -40* C. TO -90* C.; SLOWLY SCRAPING THE WALLS OF THE SCRAPED-SURFACE CHILLING ZONE AT A SCRAPING FREQUENCY OF ABOUT 5 TO 10 SCRAPES PER MINUTE TO REMOVE RELATIVELY LARGE CRYSTALS OF PARA XYLENE THEREFROM; HOLDING THE THUS SCRAPED CRYSTALS IN AN AGITATED REMAINING PORTION OF THE LIQUID FEED FOR A MINIMUM HOLDING TIME OF THIRTY MINUTES TO PERMIT FURTHER PARA XYLENE CRYSTAL GROWTH THEREFROM; AND SEPARATING THE THUS OBTAINED PARA XYLENE CRYSTALS FROM THE RESULTING MOTHER LIQUOR. 